The present invention relates to a driving technique for a stepping motor of an electronic timepiece.
In order to operate a stepping motor with less power consumption, such as an ultra micro stepping motor of an electronic wrist watch, the so called correction driving method has been proposed. The correction driving method drives a motor with a low power consumption when the stepping motor drives normally, and drives the motor with more power consumption than usual when the motor rotor fails to rotate normally for some reason or other.
When using the correction driving method, it is important to detect rotation and non-rotation of the rotor and to assure the rotor rotation against adverse external conditions, such as the influence of external magnetic fields, in comparison with the conventional fixed pulse driving method.
FIG. 1A shows an embodiment of a bipolar stepping motor of the type used both in the conventional electronic timepiece for driving the timepiece hands as well as in the present invention, and
FIG. 1B shows an example of the alternate polarity pulses conventionally used for driving the stepping motor.
By applying the driving pulse of FIG. 1B to a coil 3, a stator 1 is magnetized and a rotor 2 is rotated at 180.degree. increments by repulsion and attraction of the stator 1 and magnetic poles of the rotor 2.
Conventionally, the length (pulse width) of the driving pulse applied to the coil 3 has been determined by selecting the width needed to assure the output of the motor under any conditions of the timepiece. In order to assure the output of the motor, it was necessary to have a long enough pulse width to accommodate a calendar load, an increase in the internal resistance of the battery which occurs at low temperatures, a reduction in the batter voltage at the last stage of battery discharge, and the like, and therefore the stepping motor must be driven with driving pulses of sufficient pulse width.
Accordingly, the following driving method of the stepping motor has been proposed. Namely, normally the stepping motor is driven by a driving pulse having a short pulse width which produces a small output, and when the stepping motor stops rotating because of a heavy load, the stepping motor is driven by a pulse having a longer pulse width which produces a sufficiently large output to drive the motor under the loaded condition.
However, it was difficult to provide particular detection elements such as a mechanical contact, a Hall effect element and the like for detecting rotation and non-rotation of the rotor since a reduction in overall timepiece size and a low cost have been required.
Accordingly, the rotation and non-rotation of the rotor have been detected by taking advantage of the feature that there is a difference in voltages induced by the vibration of the rotor between the rotor being rotated and not rotated after the driving pulse is applied.
Since the conventional method for detecting rotation and non-rotation of the rotor does not operate properly when the rotor is subjected to an alternating magnetic field, it is necessary to strengthen the sealed structure of the stepping motor as compared to the conventional type.
FIG. 2 shows a driver detection circuit of the stepping motor according to the conventional type and which can also be used for carrying out the present invention. In the circuit, inputs of N channel FET gates (referred to as N gates hereafter) 4b, 5b and inputs of P channel FET gates (referred to as P gates hereafter) 4a, 5a are respectively separated and the N gates 4b, 5b and the P gates 4a, 5a are simultaneously in the OFF state while the circuit comprises detection resistors 6a, 6b for detecting rotation and non-rotation of the rotor 2 and N gates 7a, 7b for switching on the detection resistors 6a, 6b.
FIG. 3 shows a time chart of the conventional correction driving method. When a voltage is applied across the coil, a current flows in the coil through a current passage 9 in FIG. 2 during a time interval "a" in FIG. 3.
Subsequently, during a time interval "b" in FIG. 3, a current flows in the closed loop 10 which includes the detection resistor 6b in FIG. 2 by a switching operation. At this time the voltage induced by vibration of the rotor 2 appears at a terminal 8b after the driving pulse is applied. If a non-rotation signal is detected during the time interval "b", the stepping motor is driven correctly by a driving pulse of sufficiently long pulse width to cause current to again flow through passage 9 in FIG. 2 so as to satisfy the specification of the timepiece during a time interval "c" in FIG. 3.
A description will now be given of the detection principle of rotation and non-rotation of the rotor.
FIG. 4 shows current waveforms flowing in the drive coil 3 of the stepping motor, whose coil resistance is 3 K.OMEGA. and whose number of turns is 10,000 turns. When a driving pulse of 3.9 msec pulse width is applied to coil 3, the current waveform flowing through passage 9 in FIG. 2 during the time interval "a" has almost the same waveforms regardless of rotation and non-rotation of the rotor. The time interval "b" shows the current flowing in the closed loop 10 by the voltage induced under the influence of the vibration of the rotor 2 after the driving pulse is applied, varying in a large scale under the conditions of the rotor, i.e., whether the rotor rotates or not and whether a load is connected to the motor or not. The waveform b1 during the time interval "b" in FIG. 4 shows the current waveform in the case that the rotor 2 rotates and the waveform b2 shows the current waveform in the case that the rotor 2 does not rotate. The driving detection circuit in FIG. 2 has been invented to extract the difference in currents between the rotor being rotated and not rotated as a voltage waveform. During the time interval "b" in FIG. 4, the current induced by vibration of the rotor 2 flows through the detection resistor 6b in the closed loop 10 and a larger voltage waveform appears at the terminal 8b than would be the case if the detection resistor 6b were not provided. Since the current flowing in the normal direction during the time interval "b" is in the reverse direction with respect to the detection resistor 6b, a negative voltage is induced in the detection resistor 6b.
However, when the N gates 5b and 7b are in the OFF state and ON state respectively, the N gate 5b is operated as a diode by using the VSS terminal side as an anode voltage since there is a P-N junction acting as the diode between the drain and P-well of N gate 5b. Therefore the N gate 5b is biased in the forward direction by the voltage induced in coil 3 and being negative at the terminal 8b, forward current flows in the N gate 5b. And since the impedance is low in case the forward current flows in the N gate 5b, the rotor vibration is damped.
The relation between the operation of the rotor 2 and the detection signal will be illustrated in conjunction with FIG. 5. FIG. 5 shows the relation between the stator 1 and the rotor 2. FIG. 5A shows the rest condition of the rotor 2. The stator 1 is provided with inner peripheral notches 16a, 16b to determine the index torque and outer peripheral notches 15a, 15b to enable formation of a one-piece stator. In case of a two-piece stator, the stator is separated at 15a and 15b. Magnetic poles N and S rest at the positions of about 90.degree. from the inner peripheral notches 16a, 16b under the rest condition of the rotor 2.
FIG. 5B shows the condition when the driving pulse is applied to the rotor, where the rotor rotates in a direction of an arrow mark 17. Since the driving pulse width is no more than 3.9 msec, the pulse is off when the N and S poles of the rotor reach in the proximity of the inner peripheral notches 16a and 16b. In case of a heavy load, the rotor cannot complete rotation and rotates in the reverse direction as shown in FIG. 5C. In this case, the magnetic poles of the rotor 2 pass in the proximity of the outer peripheral notches 15a, 15b and a large current is generated in the coil. However, since the circuit in FIG. 2 is the closed loop 10, the negative voltage is present at the terminal 8b, and the forward current flows in the N gate 5b serving as the diode, and thereby movement of the rotor 2 is damped. Accordingly the rotor 2 is decelerated rapidly and the voltage induced by the vibration of the rotor 2 is small thereafter. On the other hand, in case of a light load and the rotor continues to rotate inertia, the rotor 2 rotates in the direction of an arrow mark 19 as shown in FIG. 5D. Since the magnetic flux generated by the rotor 2 at this time is in the direction meeting at a right angle with the outer peripheral notches 15a, 15b, the induced current is small in the beginning. And the induced current becomes large when the magnetic poles rotate to positions adjacent the outer peripheral notches 15a and 15b.
At this time, since the negative voltage is present at the terminal 8b of the closed loop 10, the rotor movement is damped by the diode effect of the N gate 5b. Thereafter the rotor passes by the rest position shown in FIG. 5A and the voltage which is used to detect the rotation of the rotor 2 is present at the terminal 8b in FIG. 2 when the rotor restores to the rest position.
Numeral 20 in FIG. 6A is the voltage waveform of the terminal 8b when the rotor 2 rotates. A time interval "a" shows a period during which the driving pulse whose pulse width is 3.9 msec is applied.
The circuit in operation during the time interval "a" is the current passage 9 in FIG. 2 whose VDD=1.57 V. A time interval "b" shows the voltage waveform of the voltage induced by the vibration of the rotor in the closed loop 10 in FIG. 2. The negative voltage is clamped at about 0.5 V by the diode effect of the N gate 5b and a peak of the positive voltage is 0.4 V. The waveform 21 shows the voltage waveform of the terminal 8b when the rotor 2 does not rotate and a peak of the positive voltage is less than 0.1 V. The rotation and non-rotation of the rotor is determined by distinguishing between these two peak voltages.
Though the difference between the two peak voltages is small, the voltage can be easily amplified by the method mentioned below.
The normally open loops 10 and 11 in FIG. 2 are alternately closed during the time interval "b" in FIG. 6A. In the loop 11, since both ends of the coil 3 are shorted by the N gates 4b, 5b having an ON resistance around 100.OMEGA., the current generated by the vibration of the rotor is large. However, when the loop 10 is switched on i.e., the loop closed), the current flows through the detection resistor 6b for an instant by an inductance constituent of the coil 3. Therefore the high peak voltage is present for an instant across the detection resistor 6b. The voltage waveform 20 at the terminal 8b induced by the rotor 2 is as shown by a voltage waveform in FIG. 6B when the normally open loops 10 and 11 in FIG. 2 are alternately closed. FIG. 6C shows the voltage waveforms 22 and 23 on an enlarged time axis. The peak voltage on this occasion delays about 30 .mu.sec from an instant that the loop 10 is closed. The delay of the peak voltage is caused by the capacitance constituent between the drain and source of the N gate 5b. The detection signals are easily amplified several times by the above mentioned method and the rotation and non-rotation of the rotor 2 can be detected much more easily. Though the rotation and non-rotation of the rotor 2 can be detected by the above mentioned method, the detection method has a great disadvantage. Namely, when the stepping motor is subjected to an external alternating magnetic field, a voltage is induced in the coil 3 by the external magnetic field and the detection resistor mistakenly judges that the rotor rotates even when it does not. Therefore, to prevent the stepping motor from malfunctioning when placed in an alternating magnetic field, the anti-magnetic characteristic must be improved so that the pulse width of 3.9 msec drives the stepping motor normally. The alternating anti-magnetic resistance is shown by curves in FIG. 7 and is less than 3 oersteds when the pulse width is 3.9 msec.
Therefore, a very close anti-magnetic structure is required to drive the stepping motor in the correction driving circuit in order to reduce the overall size, thickness and cost of the timepiece. However the advantage of the correction driving method is not fully achieved due to the space and cost taken for the anti-magnetic structure.
On the other hand, there is another driving method which varies the normal pulse width according to the load on the motor in order to reduce the current consumption in the stepping motor even more. In this case, the rotor of the stepping motor is driven by a driving pulse having the minimum pulse width needed to rotate the rotor.
As shown in FIG. 7, at smaller pulse widths the anti-magnetic characteristic deteriorates more. Accordingly, it is necessary to strengthen the anti-magnetic structure such as the sealed plate and the like. Therefore the primary object of this driving method to reduce the current in the stepping motor in order to reduce the thickness and size of the timepiece is hardly achieved.